Abstract

Acquisition of a mature pattern of gonadotropin-releasing hormone (GnRH) secretion from the CNS is a hallmark of the pubertal process. Little is known about GnRH release during sexual maturation, but it is assumed to be minimal before later stages of puberty. We studied spontaneous GnRH secretion in brain slices from male mice during perinatal and postnatal development using fast-scan cyclic voltammetry (FSCV) to detect directly the oxidation of secreted GnRH. There was good correspondence between the frequency of GnRH release detected by FSCV in the median eminence of slices from adults with previous reports of in vivo luteinizing hormone (LH) pulse frequency. The frequency of GnRH release in the late embryonic stage was surprisingly high, reaching a maximum in newborns and remaining elevated in 1-week-old animals despite low LH levels. Early high-frequency GnRH release was similar in wild-type and kisspeptin knock-out mice indicating that this release is independent of kisspeptin-mediated excitation. In vivo treatment with testosterone or in vitro treatment with gonadotropin-inhibitory hormone (GnIH) reduced GnRH release frequency in slices from 1-week-old mice. RF9, a putative GnIH antagonist, restored GnRH release in slices from testosterone-treated mice, suggesting that testosterone inhibition may be GnIH-dependent. At 2–3 weeks, GnRH release is suppressed before attaining adult patterns. Reduction in early life spontaneous GnRH release frequency coincides with the onset of the ability of exogenous GnRH to induce pituitary LH secretion. These findings suggest that lack of pituitary secretory response, not lack of GnRH release, initially blocks downstream activation of the reproductive system.

Using a technically innovative and validated technique Glanowska et
al. demonstrated GnRH release in mice during development. The prolonged
elevation in GnRH release at 1-week old mice will certainly be of a high
interest in the field. It was suggested that, rather than occurring in
response to a net driving factor, the high frequency GnRH release in 1-
week-old mice was due to a lack of inhibition from gonadotropin-inhib...

Using a technically innovative and validated technique Glanowska et
al. demonstrated GnRH release in mice during development. The prolonged
elevation in GnRH release at 1-week old mice will certainly be of a high
interest in the field. It was suggested that, rather than occurring in
response to a net driving factor, the high frequency GnRH release in 1-
week-old mice was due to a lack of inhibition from gonadotropin-inhibitory
hormone (GnIH).

Interpreting the putative GnIH effect from Glanowska et al. (2014) is
complicated by RF9, a putative antagonist. RF9 is known for its
spectacularly potent stimulatory effect on GnRH neurons and LH release,
which was recently shown to be GPR54 mediated (Liu and Herbison, 2014).
Therefore any results where RF9 stimulated GnRH release is almost
definitely not due to the blockade of GnIH secretion.

Off-target actions of RF9 aside, RF9 also lacks the assumed on-target
actions. The data from Simonin et al. (2006) showed that RF9 was unable to
completely overcome RFRP-3 induced cAMP inhibition, suggesting that RF9 is
either an extremely poor antagonist or a weak agonist. The latter is
confirmed in a separate publication where RF9 alone was clearly a NPFF1R
agonist (Findeisen et al., 2012). Importantly, these two publications are
the only pharmacological data demonstrating the function of RF9 at NPFF1R.

Therefore interpretations of a putative GnIH effect by Glanowska et
al. unfortunately suffer from the lack of specificity (or efficacy) of RF9
as a NPFF1R antagonist. For these reasons, it remains unknown why GnRH is
released at such high frequency in 1-week old male mice.